The present invention generally pertains to presence-detection-system tags that include frequency-dividing transponders and is particularly directed to deactivation of frequency-dividing transponders of the type that includes an active strip of magnetomechanical material that frequency divides when in the presence of a magnetic field within a predetermined magnetic field intensity range and a bias strip of magnetic material for biasing the active strip to be within the predetermined range.
This type of frequency-dividing-transponder is described in U.S. Pat. No. 4,727,360 to Lucian G. Ferguson and Lincoln H. Charlot, Jr., which is assigned to the assignee of the present application. According to the teaching of said patent, the frequency-dividing transponder described therein is deactivated by demagnetizing the bias strip of magnetic material. However, even after the bias strip has been demagnetized, the active strip of magnetomechanical material will still frequency divide if it is in the presence of an ambient magnetic field that is within the predetermined magnetic field intensity range. In certain locations, the ambient magnetic field resulting from the Earth's magnetic field is within the predetermined magnetic field intensity range. Presence-detection-system tags containing the above-described type of frequency-dividing transponder are adapted for attachment to articles to be detected within a surveillance zone. If the ambient magnetic field within the surveillance zone is within the predetermined magnetic field intensity range, false presence detections may occur even after the bias strip has been demagnetized.